Spark ignition engines for automobiles, for example, have heretofore employed a carburetor system in which fuel is sucked in and atomized to mix with air in a carburetor by means of a negative air pressure that is produced by the flow of intake air, or a pressure injection valve system in which a liquid fuel is injected from a nozzle under pressure and the fuel thus atomized is mixed with air. The fuel-air mixture produced in either way is then carried to a combustion chamber by a stream of air flowing at a high velocity, where it is burned by spark ignition. The above-described fuel-air mixture is in a state where droplets of fuel are suspended in mist-like form in a high-velocity air stream. Although part of the fuel is in the form of vapor, the greater part of it adheres to the wall of the flow path and forms into a liquid, which is sucked into a cylinder through an intake pipe by the pressure of the air stream. During this process, the fuel in the liquid form is evaporated by the heat from the wall surface of the flow path or the heat in the cylinder. Thus, since the greater part of the fuel evaporates while being delivered in the form of a liquid flow on the wall surface, the injected fuel cannot promptly be delivered into the cylinder, so that the engine response and the combustion efficiency are not always satisfactory. In particular, at the time of starting the engine, the wall surface of the intake pipe is dry and consequently the greater part of the fuel injected adheres to the wall surface and fails to reach the combustion chamber. Thus, the above-described conventional systems suffer from inferior startability.
To cope with this problem, electronically controlled injection engines have heretofore adopted a control method wherein a pressure injection valve is controlled with a computer such that the supply of fuel is incremented according to a predetermined increment ratio pattern (in which the supply of fuel in steady-state running is determined to be 1), thereby striving to improve the startability. More specifically, the increment ratio is maintained at a constant level while the starter is in an operative state, and after the starter has been turned off, the increment ratio is reduced at a given rate in accordance with the temperature of a coolant. In carburetor engines, the increment control of the supply of fuel is effected by a choke mechanism to improve the startability. In this system, however, an oversupply of fuel occurs during and immediately after the starting of the engine, resulting in a rise in the fuel consumption rate and an increase in exhaust emissions (HC, CO, etc.).
In low-temperature (cold) conditions, fuel increment control for warming up is carried out according to a pattern in which the increment ratio is increased in accordance with the lowering in the coolant temperature to compensate for the deterioration of the operating characteristics due to lowering in the vaporability of gasoline in the intake pipe. In this case also, an oversupply of fuel causes similar problems to those in the fuel increment control at the time of starting the engine.
FIG. 1 shows the results of an experiment in which the above-described fuel increment control for starting was carried out with the same increment ratio pattern for an engine equipped with a conventional pressure injection valve and an engine equipped with an ultrasonic atomizer (described later).
As will be clear from the figure, in the engine equipped with the ultrasonic atomizer the time required to reach steady-state running shortens by about 35% of that in the engine equipped with the pressure injection valve mainly because of the reduction in the idling time, but there is substantially no reduction in the cranking time (i.e., the period of time during which the starter is ON).
Similarly, an engine equipped with a conventional pressure injection valve and an engine equipped with an ultrasonic atomizer (described later) were subjected to the fuel increment control for warming up at an ambient temperature of -20.degree. C., with the throttle valve full open and with the gear shifted at an optimal timing to examine accelerability based on the speed change. The results are shown in FIGS. 2(a)-2(b), in which the solid line shows the results for the ultrasonic atomizer, and the chain line shows those for the pressure injection valve.
During the first five minutes, in which the coolant temperature has not yet reached 50.degree. C., the engine equipped with the conventional pressure injection valve is better in accelerability, and at about 60.degree. to 70.degree. C., the accelerability becomes substantially constant.
Thus, no adequate operating characteristics can be obtained if the engine equipped with the ultrasonic atomizer is subjected to fuel increment control for starting and warming up with the same patterns as those for the engine equipped with the conventional pressure injection valve.
On the other hand, in the ultrasonic atomizer the fuel is substantially completely atomized when injected and is mixed with air to form a fuel-air mixture and efficiently delivered into the cylinder by an air stream in this state, so that the combustion efficiency is high. In addition, if the fuel injection is carried out in a pulsational manner and the injection frequency or duty is properly varied, the response of the engine can be improved.
Incidentally, with the recent strict regulation of exhaust emissions (HC, CO, etc.), attempts have been made to utilize alcohols such as methanol and ethanol as fuel, and spark ignition engines have been proposed which use, for example, a fuel consisting of 100% of methanol or ethanol, or an alcohol-gasoline mixture which contains not less than 50% of alcohol. Methanol and ethanol are superior from the environmental point of view, but the flash points of these fuels are high in comparison to gasoline, i.e., 11.degree. C. and 13.degree. C., and the latent heat of vaporization of these fuels is relatively large. Therefore, if the engine is left to stand for a long time and the temperature in the combustion chamber becomes lower than the flash point of these fuels, the engine cannot be started. Thus, this type of engine has the disadvantage of inferior startability. To overcome this problem, Japanese Patent Laid-Open (KOKAI) No. 57-153964 (1982) proposes a method wherein an intake pipe of an engine is provided with an ultrasonic vibration type spray nozzle and a surface heating element which reflects the spray from the nozzle to form a mist of fine droplets, and at the time of starting the engine, an alcohol fuel is atomized by the spray nozzle and the surface heating element, and after the engine has been started, the alcohol fuel is supplied through a carburetor. In this method, however, the ultrasonic spray nozzle and the surface heating element must be provided merely for the starting of the engine, which is not very frequently performed, and the cost increases correspondingly.
Conventional ultrasonic atomizers will next be explained with reference to FIGS. 3 and 4.
FIG. 3 shows a multihole ultrasonic injection valve of the type that a liquid is supplied to an atomization surface from a plurality of nozzle holes. The ultrasonic injection valve comprises a cylinder 101, a nozzle body 102, a vibrator horn 103 and an electroacoustic transducer 104. The cylinder 101 is formed with a fuel feed passage 105, and the nozzle body 102 is provided with a plurality of nozzle holes 106 which are communicated with the fuel feed passage 105, the nozzle holes 106 being circumferentially formed in the nozzle body 102 so that fuel which is injected from the nozzle holes 106 is supplied to the vibrator horn 103 where it is atomized.
FIG. 4 shows an annular ultrasonic injection valve of the type that a liquid is supplied to an atomization surface from a ring-shaped groove. This ultrasonic injection valve comprises an outer cylinder 111, an inner cylinder 112, a vibrator horn 113 and an electroacoustic transducer 114. A fuel feed passage 115 is formed in between the outer cylinder 111 and the inner cylinder 112, so that fuel is supplied to the vibrator horn 113 from the entire circumference of the outer cylinder 111 and thus atomized on the horn surface.
Incidentally, it is essential in alcohol engines to form a thin film of liquid uniformly over the atomization surface of the vibrator in order to ensure an excellent atomization efficiency over a wide fuel supply range. It is also important, in order to atomize the whole amount of fuel supplied, to prevent the fuel from being splashed on the atomization surface even when the fuel feed velocity is high.
However, in the multihole ultrasonic injection valve stated above, the quantity of atomized fuel is determined by the quantity of fuel supplied from the nozzle holes 106 and it is therefore impossible to obtain a high turn-down ratio that represents the ratio of the maximum atomization quantity to the minimum atomization quantity. When the injection valve is used in a horizontal position, it is difficult to distribute the liquid uniformly among the nozzle holes 106 and the resulting spray becomes nonuniform. If the number of nozzle holes 106 is increased, the fuel may be distributed uniformly. However, the number of nozzle holes 106 which can be provided is limited, and since it is difficult to form a large number of nozzle holes 106 by machining process, the production cost increases.
In the annular ultrasonic injection valve, the atomization quantity is determined by the clearance 116 between the tip of the outer cylinder 111 and the vibrator horn 113. Accordingly, a high degree of accuracy is required to mount the outer cylinder 111 to the collar portion 113a of the vibrator horn 113, which leads to an increase in the production cost. If the clearance 116 cannot be provided with adequate tolerances, a high turn-down ratio cannot be obtained, and the resulting spray becomes nonuniform. In addition, the above-described prior art involves the problem that the spray angle of the fuel atomized by the ultrasonic injection valve is relatively large and the fuel is likely to adhere to the inner wall of the intake pipe, which has a relatively small diameter.
Thus, in the ultrasonic atomizer, the film of a liquid fuel injected flows along the horn surface and scatters in the form of liquid droplets from the horn tip. The size of liquid droplets formed at that time is related to the thickness of the liquid film flowing along the horn surface, that is, the thicker the liquid film, the larger the droplet diameter, and vice versa. Accordingly, when the fuel injection is carried out in a pulsational manner, the thickness of the liquid film varies periodically and the droplet diameter periodically increases and decreases in response to the change in the film thickness. When the droplet diameter is large, the droplets are likely to adhere to the wall surface of the intake pipe and hence cannot effectively mix with air. Therefore, the engine cannot readily be ignited, and the startability deteriorates, particularly in low-temperature conditions. The deterioration of the startability is particularly noticeable in automotive engines of the SPI (Single Point Injector) type in which fuel feed is performed in the vicinity of a carburetor to distribute the fuel to a plurality of cylinders.
In addition, when an alcohol fuel is used, the cold startability is not good even if an ultrasonic atomizer is employed, as stated above.
Unlike the conventional system wherein fuel is sucked in by means of an intake air stream, the fuel injection system that employs an ultrasonic atomizer is capable of conducting fuel injection independently of the air stream. Therefore, no satisfactory explanation has yet been given about a condition of air stream which is suitable for efficient injection of fuel.